Why We Have Ice Ages

Ice ages come and ice ages go. Every 100,000 years or so there has been a predictable warming period during which polar glaciers melt and sea levels rise. Then another ice age arrives and the process repeats.

There are more than 30 theories as to what is behind these shifts, but a pair of Woods Hole and MIT

climatologists may have worked out the final answer. Peter Huybers and Carl Wunsch compared the timing of the last seven thaws, as determined from sediment records, with previously calculated changes in Earth’s orbit. They found that the planet’s slant has a major impact. “Earth is tilted on its axis, but it’s not always the exact same tilt,” explains Huybers. The axis goes up and down a few degrees in 40,000-year cycles. “When the tilt is at its highest, more sunlight hits the higher latitudes, melting the ice.”

How does that account for ice ages every 100,000 years? “What we think is happening is that the glaciers have to get large enough for the increased sunlight to have an effect, so the thaws happen every second or third cycle, which would average out to 100,000 years,” Huybers says. “This makes sense because if you go back another million years, ice ages were occurring every 40,000 years.” Earth has cooled since then, allowing the cycle to skip a beat.

Could the cycles help explain today’s global warming? Not really, says Huybers. “Global warming is a very recent phenomenon, and the last big thaw was only 20,000 years ago, so if anything we should be heading toward another ice age.”